1. Field of the Invention
This invention relates to an optical type surface defect inspection apparatus for a transparent substrate such as glass, and particularly to a foreign particle inspection apparatus for the surface of a mask such as a reticle used in the semiconductor manufacturing process.
2. Related Background Art
U.S. Pat. No. 4,468,120 discloses an apparatus of this kind. In this prior-art apparatus, a light beam such as a laser beam emitted from a light source is swept by a scanner such as a vibratory mirror and enters a scanning lens. The light beam is applied as spotlight L onto the surface of a reticle by the scanning lens, and scans any given range. With the scanning direction of this spotlight L as the X-axis, the reticle is moved in the direction of the Y-axis substantially orthogonal to the X-axis, whereby the whole surface of the reticle can be scanned.
If there is any defect such as a foreign particle on the surface of the reticle at this time, scattered light is created by the spotlight L being applied to the foreign particle and is photoelectrically detected by a plurality of light receiving elements. Now, such scattered light is created not only from the foreign particle, but also as pattern scattered light or diffracted light from the edge of a circuit pattern depicted on the reticle. Incidentally, the pattern scattered light or the diffracted light has high directionality and can be distinguished from foreign particle scattered light of low directionality.
However, when such pattern scattered light or diffracted light enters the light receiving elements, it will be detected as scattered light from the foreign particle and accurate particle detection will become impossible. So, the plurality of light receiving elements are disposed so as to receive scattered light or diffracted light from different directions (in other words, so that all of the plurality of light receiving elements may not receive pattern scattered light or diffracted light of high directionality at one time). Accordingly, it can be judged that if scattered light enters all of the plurality of light receiving elements, this scattered light is that from the foreign particle and if there is any one of the plurality of light receiving elements which scattered light or diffracted light does not enter, this scattered light or diffracted light is that from the edge of the pattern.
These light receiving elements are disposed at an angle far from the direction in which pattern diffracted light of low order is created in order to make it difficult to receive low-order pattern diffracted light of high luminance. This also is a disposition which makes it difficult to receive pattern scattered light of high luminance.
Further, it is also known to use an apparatus as disclosed in U.S. Pat. No. 4,568,835 in order to prevent light (stray light) created on and in a reticle when light is applied to the reticle from entering light receiving elements. In this apparatus of a construction wherein a light beam emitted from a laser source is obliquely applied to the surface of a reticle and the light beam and the reticle are scanned relative to each other and scattered light from a foreign particle on the surface of the reticle is received by a light receiving device disposed in the space over the surface of the reticle, a light absorbing member extending in the scanning direction of the light beam is disposed in the direction in which scattered light (stray light) appearing from the surface or end surface of the reticle is created, and a light intercepting member is disposed in such a direction which does not intercept the optical path of the light beam and the optical path along which the scattered light from the foreign particle arrives at the light receiving element. This light intercepting member has intercepted stray light resulting from part of the laser light which enters the reticle being repetitively reflected within the reticle and travelling away from the laser source and being reflected by an end surface of the reticle. The erroneous detection by the foreign particle inspection apparatus caused by stray light has been reduced by this light intercepting member.
However, even the use of the prior-art light intercepting member could not completely intercept stray light.